Multiplication takes place by division (Marchand). Encysted forms are almost unknown.
_Trichomonas vaginalis_ lives in the vaginal mucus of women of various ages, not in normal mucus, but in mucus of acid reaction. It is found in menstruating females as well as in females who have passed the menopause. It occurs in pregnant and non-pregnant women, even in very young girls, provided always that they have a vaginal catarrh with acid reaction of the secretion. Should the acid reaction change, as, for instance, during menstruation, the parasites disappear, as they do likewise on injection of any alkaline fluid into the vagina. A low temperature (below +15° C.) is also fatal to the parasites. These flagellates can pass from the vagina through the urethra into the bladder, and produce severe catarrh, and are not easily removed.
[Illustration: FIG. 16.--_Trichomonas vaginalis_, Donné. × 2,000 approx. (After Künstler.) Four flagella are represented, but usually only three are present.]
_T. vaginalis_ appeared to be a parasite specific to the female organs and not transmissible to man. However, several observations have since been made that confirm the occurrence of this species in the urethra of the male. The infection apparently takes place through coitus when changes are present in the urethral mucous membrane. At any rate, three cases observed point to this circumstance.
Attempts at experimental transmission to rabbits, guinea-pigs and dogs failed (Blochmann, Dock). So far, the manner in which women become infected is unknown.
*Trichomonas intestinalis*, R. Leuckart, 1879 = *Trichomonas hominis*, Davaine, 1854.
Some authors believe that a second trichomonad inhabiting man, _Trichomonas intestinalis_, R. Lkt., is identical with _Trichomonas vaginalis_, Donné. Leuckart’s species was based on the discoveries of Marchand (1875) and Zunker (1878), who stated that according to all appearances, and in their opinion, it was the same as _Cercomonas intestinalis_, Lambl, 1875 (_nec_ 1859), which they found in the fæces of patients suffering from intestinal disorders. The organism is described by them as being pear-shaped and 10 µ to 15 µ in length and 3 µ to 4 µ in breadth. The posterior extremity terminated in a point (fig. 17).